Method for Carrying Out combined Burning in a Recovering Furnace

ABSTRACT

The invention relates to a burning method carried out in furnace provided with energy recovering means and burners, wherein a part of burners are embodied in the form of aero-combustible burners and the other part thereof are embodied in the form of oxy-combustible burners which are placed under the air ducts of the aero-combustible burners and carry out a staged combustion method.

The present invention relates to a method of combustion in a furnace provided with energy recovery means.

Regenerative furnaces are furnaces equipped with stacks of refractories along their sides. These refractories are heat exchangers for recovering the heat from the flue gases leaving the sides of the furnace and for transferring this heat to cold air supplied to the furnace. The refractories of the regenerators are heated to very high temperatures by the flue gases (about 1300 to 1500° C.). In practice, the flue gases leaving via one side of the furnace come into contact with the refractories from their upper part down to their lower part during a cycle, generally lasting about 20 minutes. During the next cycle, the cold combustion air supplied to the burners of the furnace comes into contact the refractories from their lower part to their upper part in order to extract the heat therefrom. The combustion air is then heated to a temperature generally of about 1100 to 1300° C. before being introduced into the furnace combustion chamber. The streams of flue gases and combustion air are reversed at each cycle so that each side of the regenerator can be heated alternately and used to preheat the combustion air. Preheating the combustion air allows for air combustion with a high energy efficiency. For regenerative furnaces, the combustion air is heated continuously by metal heat exchangers fed with flue gases. These regenerative furnaces operate with air-fuel burners (or air burners), that is, burners in which the oxidizer is air. This is also referred to as air-combustion.

The service life of these furnaces is generally about 10 to 15 years. During a run, the basic problem consists in maintaining the production capacity of the furnace despite the increasing wear of the refractories and of the regenerators. This problem may even prove to be more critical in case of a need to increase production beyond the nominal capacity of the furnace, in order to contend with changes in market requirements over time.

Various alternatives are available for maintaining or increasing the production capacity of an existing regenerative furnace. Firstly, it is possible to supplement the heating electrically by installing electrodes through the floor or the side walls of the glass melt tank. This solution has the intrinsic advantage of great flexibility (production capacity can be increased by 10 to 15%) but its implementation is problematic and there is no guarantee of results, because the choice of the power distribution and electrode positioning is empirical (there is no accurate model of the effect of the electrodes on the glass melt). It is also difficult to position hot electrodes (especially if the locations have not been provided for before the construction of the furnace). The investment cost is high (transformers) and the price of electric power significantly higher than that of the fossil fuels conventionally used for glass melting.

Another solution consists in partially converting the air-combustion carried out in the furnace to oxy-combustion (in the context of the present invention, oxy-combustion means combustion using an oxidizer comprising oxygen-containing gas that is richer in oxygen than air). Oxy-combustion can be carried out by using one of the following technologies:

-   -   the addition of oxy-burners (or oxy-fuel burners) through the         roof of the furnace in the melting zone (in the context of the         present invention, oxy-burners or oxy-fuel burners means burners         in which the oxidizer is an oxygen-containing gas that is richer         in oxygen than air);     -   or the partial or total conversion of air-burners to         oxy-combustion.

However, these two solutions have drawbacks. The addition of oxy-burners through the furnace roof gives rise to intrinsic difficulties in the positioning of the oxygen burners in the roof: the placement of this type of burner during a run (roof drilling) is problematic. Excessive thermomechanical stresses (in the burner blocks and noses (high pressure and temperature) or of the roof itself (embrittlement)) may cause faster wear of the materials and equipment and, in consequence, more flaws in the glass. An increase is also observed in the dust fly-off from the batch blanket covering the melt in the melting zone (impacting flames) and also the disturbance of the redox state of the glass (greater presence of CO in contact with the glass in the melting zone) which is liable to cause flaws in the glass.

As to the partial or total conversion of air-burners to oxy-combustion, this is mainly carried out by placing oxygen lances under an air duct while adjusting the rate of hot air flow introduced via the duct concerned or by adding oxy-burners on each side of the furnace. The following limitations are observed to these conversions to oxygen currently available on regenerative furnaces:

-   -   a significant increase in the temperatures of the roof (+30 to         50° C.) and the superstructure (close to the oxygen use         locations), potentially detrimental to the wear and integrity of         the furnace, and also to the quality of the glass (“refractory         stone” type flaws). This drawback is particularly pronounced in         the case of partial conversions to oxygen for which a         significant increase in the flame root temperatures (and even in         the duct) is observed (faster combustion resulting from the         higher quantity of oxygen in the flame and from the higher         temperature of the air preheated in the regenerators),     -   insufficient increase in the production of the furnace (output):         -   no guarantee of higher output in case of partial port             conversion (virtually systematic flame shortening             detrimental to heat transfer to the melt due to excessive             flame root heating and poorer coverage of the melt by the             flames),         -   slight increase in case of oxy-fuel burners added on each             side of the furnace, this increase being limited by the size             of the oxygen flames developed and the maximum power             injectable into the zone concerned,     -   risk of negative interaction with the flames produced by         air-combustion:         -   flame deviation detrimental to the wear of the refractories             and the efficiency of heat transfer to the batch,         -   NOx emissions difficult to control (in kg NOx/tonne of             melt),         -   higher emissions in case of partial port conversion (hotter             and shorter flames),         -   at best, stabilization of the emissions in case of total             conversion of a port (compared to an air port) or in the             case of oxygen burners added to the furnace and coexisting             with air ports.     -   difficulty of placing conventional oxygen burners under the duct         of a port, difficult and even impossible to carry out         industrially owing to the lack of space under the air duct.

It is one object of the present invention to provide a method for increasing the capacity (glass output and quality) of the regenerative furnaces using an oxy-combustion technology, a method that is easy to implement.

Another object is to provide a method for increasing the capacity of regenerative furnaces using an oxy-combustion technology suitable for controlling the NOx emissions in the flue gases in comparison with existing technologies.

A further object is to provide a method for adapting the combined air- and oxy-fuel combustion in regenerative furnaces without altering the profile of the air-fuel flame.

For this purpose, the invention relates to a combustion method carried out in a furnace provided with energy recovery means and burners, in which:

-   -   the heat of the flue gases is alternately recovered by one part         of the energy recovery means and then by the other part of the         energy recovery means,     -   at least some of the burners are air-fuel burners consisting of         at least one air duct under which or at the center of which at         least one injector of a first fuel is placed, said injector         being perpendicular to the furnace wall,     -   two phases are alternately implemented:         -   a first phase in which only one part of the energy recovery             means operates and only the air-burners capable of sending             their flames to said operating energy recovery means are in             operation,         -   a second phase in which only the other part of the energy             recovery means operates and only the air-burners capable of             sending their flames to said operating energy recovery means             are in operation, and in which at least one oxy-fuel burner             is placed under the air duct of at least one air-fuel             burner, said oxy-fuel burner comprising fluid injectors             perpendicular to the furnace wall and implementing a method             of staged combustion of a second fuel and of an oxygen-rich             gas, said staged combustion method being carried out in such             a way that at least one jet of the second fuel and at least             one jet of oxygen-rich gas is injected, the jet of             oxygen-rich gas comprising a primary jet of oxygen-rich gas             and a secondary jet of oxygen-rich gas,             -   the primary jet of oxygen-rich gas being injected close                 to the jet of the second fuel in order to cause a first                 incomplete combustion, the gases issuing from this first                 combustion still comprising at least part of the second                 fuel, while the secondary jet of oxygen-rich gas is                 injected at a distance I₂ from the jet of the second                 fuel which is greater than the distance between the jet                 of the second fuel and the primary jet of oxygen-rich                 gas closest to the jet of the second fuel, in order to                 enter into combustion with the part of the second fuel                 present in the gases issuing from the first combustion,                 the primary jet of oxygen-rich gas being divided into at                 least two primary jets: at least a first primary jet of                 oxygen-rich gas which is injected in contact with the                 jet of the second fuel, and at least a second primary                 jet of oxygen-rich gas injected at a distance I₁, from                 the jet of the second fuel, where I₂ is greater than I₁.

The invention also relates to a system for carrying out a combustion in a furnace comprising:

-   -   at least one air-fuel burner composed of:         -   an air duct, and         -   at least one fuel injector placed under or at the center of             the air duct and perpendicular to the furnace wall,     -   at least one oxy-fuel burner placed under the air duct of the         air-fuel burner and composed of at least one set of injectors         comprising:         -   at least one fuel injector,         -   at least one oxygen-rich gas injector placed so as to inject             said oxygen-rich gas in contact with the fuel injected by             the injector of the oxy-fuel burner,         -   at least one oxygen-rich gas injector placed at a distance             I₁ from the fuel injector of the oxy-fuel burner,         -   at least one oxygen-rich gas injector at a distance I₂ from             the fuel injector of the oxy-fuel burner, where I₂>I₁,             said injectors being perpendicular to the furnace wall.

Other features and advantages of the invention will appear from a reading of the following description. Alternative embodiments of the invention are provided as nonlimiting examples, illustrated by the appended drawings in which:

FIG. 1 is a schematic view of a furnace equipped with lateral energy recovery means,

FIG. 2 is a schematic view of a furnace equipped with energy recovery means at the batch charging level,

FIG. 3 is a schematic view of the combustion system according to the invention.

The invention therefore primarily relates to a combustion method carried out in a furnace provided with energy recovery means and burners, in which:

-   -   the heat of the flue gases is alternately recovered by one part         of the energy recovery means and then by the other part of the         energy recovery means,     -   at least some of the burners are air-fuel burners consisting of         at least one air duct under which or at the center of which at         least one injector of a first fuel is placed, said injector         being perpendicular to the furnace wall,     -   two phases are alternately implemented:         -   a first phase in which only one part of the energy recovery             means operates and only the air-burners capable of sending             their flames to said operating energy recovery means are in             operation,         -   a second phase in which only the other part of the energy             recovery means operates and only the air-burners capable of             sending their flames to said operating energy recovery means             are in operation, and in which at least one oxy-fuel burner             is placed under the air duct of at least one air-fuel             burner, said oxy-fuel burner comprising fluid injectors             perpendicular to the furnace wall and implementing a method             of staged combustion of a second fuel and of an oxygen-rich             gas, said staged combustion method being carried out in such             a way that at least one jet of the second fuel and at least             one jet of oxygen-rich gas is injected, the jet of             oxygen-rich gas comprising a primary jet of oxygen-rich gas             and a secondary jet of oxygen-rich gas,             -   the primary jet of oxygen-rich gas being injected close                 to the jet of the second fuel in order to cause a first                 incomplete combustion, the gases issuing from this first                 combustion still comprising at least part of the second                 fuel, while the secondary jet of oxygen-rich gas is                 injected at a distance I₂ from the jet of the second                 fuel which is greater than the distance between the jet                 of the second fuel and the primary jet of oxygen-rich                 gas closest to the jet of the second fuel, in order to                 enter into combustion with the part of the second fuel                 present in the gases issuing from the first combustion,                 the primary jet of oxygen-rich gas being divided into at                 least two primary jets: at least a first primary jet of                 oxygen-rich gas which is injected in contact with the                 jet of the second fuel, and at least a second primary                 jet of oxygen-rich gas injected at a distance I₁ from                 the jet of the second fuel, where I₂ is greater than I₁.

The furnace of the method according to the invention is equipped with energy recovery means used to heat cold air, that is ambient air, by transferring to it the energy recovered from the flue gases. These energy recovery means are placed either on each side of the furnace on the sides thereof, or at the end of the furnace where the batch is charged.

The furnace is also equipped with air-fuel burners. In the context of the present invention, “air-fuel burners” means burners implementing the combustion of air and of a fuel. According to the present method, an air-fuel burner consists of at least one air duct under which or at the center of which at least one fuel injector is placed. The injector is perpendicular to the furnace wall in order to create a flame substantially perpendicular to the furnace wall. However, angle effects may be imparted to the flame. The heat liberated by the flue gases of the air-fuel burners is recovered by the energy recovery means in operation, the operating mode corresponding to the phase in which the energy recovery means recover the heat, while the shutoff mode corresponds to the case in which the energy recovery means give up their heat to cold air. The energy recovery means and the air-fuel burners operate in phase opposition: those air-fuel burners which do not direct their flames to the operating energy recovery means are extinguished, while those air-fuel burners which direct their flame and their flue gases to the operating energy recovery means are in operation.

According to the invention, the method also uses oxy-fuel burners placed under the air duct of at least one air-fuel burner. In the context of the present invention, oxy-fuel burner means a burner using the combustion of a fuel and of an oxygen-rich gas, that is a gas having an oxygen concentration higher than 90% by volume. The oxygen produced by a VSA (vacuum swing adsorption) process is ideal for this purpose. The fuel of the oxy-fuel burners may be identical to or different from that of the oxy-fuel burners. It is understood that the oxy-fuel burners are under an air duct but remain above the melt. The method can use an oxy-fuel burner under an air duct or a plurality of oxy-fuel burners under one or more air ducts. If the furnace is equipped with air ducts placed in the sides, in that case the method preferably uses an even number of oxy-fuel burners placed symmetrically on each side under opposing air ducts. The oxy-fuel burners used according to the invention are of a particular type: they must implement a staged combustion of the fuel that they burn, the oxygen-rich gas being injected in the form of at least three different jets: injection of a first primary jet in contact with the fuel, injection of a second primary jet injected at a distance I₁ from the fuel injection point, and injection of a secondary jet injected at a distance I₂. Within the context of the present invention, the expression “injection in contact with the fuel” means the fact that one of the primary jets is injected either at the center of the jet of second fuel, or in the form of a sheath around the jet of second fuel, the distance between the first primary jet of oxygen-rich gas and the second fuel therefore being zero. This type of oxy-fuel burner is described in particular in applications WO 02/081967, WO 2004/094902 and WO 2005/059440. The injectors of the oxy-fuel burner are perpendicular to the furnace wall in order to create a flame substantially perpendicular to the furnace wall. However, angle effects may be imparted to the flame. The flame created by the oxy-fuel burner is substantially parallel to that created by the air-fuel burner.

If the furnace is a melting furnace in which the energy recovery means are placed on the sides, then preferably at least one oxy-fuel burner is placed under an air duct located in the furnace melting zone.

When an oxy-fuel burner is placed under an air duct of an air-fuel burner, then the flow rate of oxygen-rich gas injected by the oxy-fuel burner is generally between 20 and 100% of the flow rate of oxygen-rich gas and air injected by said burner and the air duct under which it is placed. The 100% flow rate corresponds to the phase in which the air-fuel burner is extinguished.

Preferably, the combustion carried out by the air-fuel burners is substoichiometric and the combustion carried out by the oxy-fuel burners is superstoichiometric.

According to the invention, it is possible to control the combustion method, and in particular the combustion carried out in the oxy-fuel burners, according to the heat recovery cycles of the energy recovery means. Thus, according to a first alternative, for each oxy-fuel burner placed under the air duct of an operating air-fuel burner, the flow rate of oxygen-rich gas in the secondary jet of oxygen-rich gas represents 70 to 80%, preferably 75%, of the total quantity of oxygen-rich gas injected by said oxy-fuel burner. This distribution of oxygen-rich gas in the secondary oxidizing gas injectors of the oxy-burners serves to provide a broad air and oxygen flame.

According to a second alternative, for each oxy-fuel burner placed under the air duct of an operating air-fuel burner, the flow rate of oxygen-rich gas in the secondary jet of oxygen-rich gas represents 45 to 55%, preferably 50%, of the total quantity of oxygen-rich gas injected by said oxy-fuel burner. This distribution of oxygen-rich gas in the secondary oxidizing gas injectors of the oxy-burner serves to provide a stable air and oxygen flame and to concentrate the heat transfer to the melt placed near the root of the flame.

As to the operation of the oxy-fuel burners, two operating embodiments of the method can be implemented.

According to a first embodiment of the method, only the oxy-fuel burners whose flames are directed at the operating energy recovery means are also in operation. In this embodiment, the oxy-fuel burners operate in parallel to the air-fuel burners: when part of the energy recovery means recover the flue gases from the operating air-burners, the oxy-burners placed under the air duct of these air-fuel burners also operate, and when the latter energy recovery means are extinguished, then these air-burners which supplied heat to them are also extinguished and the oxy-burners placed under the air ducts of these extinguished air-fuel burners are also extinguished.

In a second embodiment of the method, the oxy-fuel burners operate permanently, independently of the operating and shutoff phases of the heat recovery means and the air-fuel burners. During the implementation of this second embodiment, according to a first alternative, for each oxy-fuel burner placed under the air duct of a stopped air-fuel burner, the flow rate of oxygen-rich gas in the secondary jet of oxygen-rich gas can represent 70 to 80%, preferably 75%, of the total quantity of oxygen-rich gas injected by said oxy-fuel burner. This distribution of oxygen-rich gas in the secondary oxidizing gas injectors of the oxy-burners serves to reduce the NOx emissions of the flames and to provide a broad flame.

During the implementation of this second embodiment, according to a second alternative, for each oxy-fuel burner placed under the air duct of a stopped air-fuel burner, the flow rate of oxygen-rich gas in the secondary jet of oxygen-rich gas can represent 45 to 55%, preferably 50%, of the total quantity of oxygen-rich gas injected by said oxy-fuel burner. This distribution of oxygen-rich gas in the secondary oxidizing gas injectors of the oxy-burners serves to reduce the NOx emissions of the flames and to increase the temperature of the flue gases.

The invention also relates to a system for carrying out a combustion in a furnace comprising:

-   -   at least one air-fuel burner composed of:         -   an air duct, and         -   at least one fuel injector placed under or at the center of             the air duct and perpendicular to the furnace wall,     -   at least one oxy-fuel burner placed under the air duct of the         air-fuel burner and composed of at least one set of injectors         comprising:         -   at least one fuel injector,         -   at least one oxygen-rich gas injector placed so as to inject             said oxygen-rich gas in contact with the fuel injected by             the injector of the oxy-fuel burner,         -   at least one oxygen-rich gas injector placed at a distance             I₁ from the fuel injector of the oxy-fuel burner,         -   at least one oxygen-rich gas injector at a distance I₂ from             the fuel injector of the oxy-fuel burner, where I₂>I₁,             said injectors being perpendicular to the furnace wall.

The oxy-fuel burner may be selected from those described in applications WO 02/081967, WO 2004/094902 and WO 2005/059440.

In general, in the combustion system according to the invention, the fuel injector of the air-fuel burner is placed under the air duct and said injector and the injectors of the oxy-fuel burner are placed substantially in the same horizontal plane.

Preferably, the oxy-fuel burner is composed of two sets of injectors, said sets being arranged symmetrically about the center of the air duct under which the burner is placed. In this preferred system, it is advantageous to place at least one fuel injector of the air-fuel burner between the two sets of injectors of the oxy-fuel burner. According to a preferred embodiment, the oxy-fuel burner comprises at least two fuel injectors and said fuel injectors are placed on each side of the two sets of injectors of the oxy-fuel burner.

FIGS. 1, 2 and 3 illustrate the device and the method according to the invention.

FIG. 1 illustrates the operation of a furnace equipped with air-fuel burners and lateral energy recovery means. The energy recovery means (1 and 11) are placed laterally on the sides of the furnace 6. Air-fuel burners (not shown) produce flames 2: the flames of these burners are directed at the energy recovery means 11 in operation, while the energy recovery means 1 placed on the same lateral side of the furnace 6 as the operating burners are extinguished.

FIG. 2 illustrates the operation of a furnace equipped with air-fuel burners and energy recovery means (1, 11) placed at the end of the furnace (6) where the batch is charged. An air-fuel burner (not shown) produces a flame 2 having a return movement to the end of the furnace (6) where the batch is charged. The energy recovery means 11, to which the flame 2 is directed, are in operation, while the energy recovery means 1 placed behind the operating burner are extinguished.

FIG. 3 illustrates a combustion system according to the invention composed of:

-   -   an air-fuel burner comprising an air duct 3 and three fuel         injectors 4 placed under the air duct,     -   an oxy-fuel burner comprising two sets 5 of injectors, each set         being placed symmetrically under the air duct 3. One fuel         injector 4 of the air-fuel burner is placed between the two sets         5 of injectors of the oxy-fuel burner. The other two fuel         injectors 4 of the air-fuel burner are placed around the two         sets 5 of injectors of the oxy-fuel burner.

By implementing the method and the combustion system according to the invention as previously described, it is possible to increase the capacity of a furnace equipped with energy recovery means and air-fuel burners and thereby to increase the production capacity of the furnace.

The method and the combustion system according to the invention also serve to carry out combustions of fuels of different types according to the nature of the air- or oxy-fuel burners.

The second embodiment of the method according to the invention (in which oxy-fuel burners operate permanently, independently of the operating and shutoff phases of the heat recovery means and the air-fuel burners) serves to maintain the heating of the melt. Due to the small volume of flue gases created by the oxy-fuel burners, it is in fact unnecessary to extinguish them when the heat recovery means change their operating mode. In this case, the power of the oxy-fuel burners can be adjusted to compensate for the asymmetrical heating of the air-fuel burners. 

1-13. (canceled) 14: A combustion method carried out in a furnace provided with energy recovery means and burners, in which: a) the heat of the flue gases is alternately recovered by one part of the energy recovery means and then by the other part of the energy recovery means; b) at least some of the burners are air-fuel burners consisting of at least one air duct under which or at the center of which at least one injector of a first fuel is placed, said injector being perpendicular to the furnace wall; and c) two phases are alternately implemented: 1) a first phase in which only one part of the energy recovery means operates and only the air-burners capable of sending their flames to said operating energy recovery means are in operation; and 2) a second phase in which only the other part of the energy recovery means operates and only the air-burners capable of sending their flames to said operating energy recovery means are in operation, wherein at least one oxy-fuel burner is placed under the air duct of at least one air-fuel burner, said oxy-fuel burner comprising fluid injectors perpendicular to the furnace wall and implementing a method of staged combustion of a second fuel and of an oxygen-rich gas, said staged combustion method being carried out in such a way that at least one jet of the second fuel and at least one jet of oxygen-rich gas is injected, the jet of oxygen-rich gas comprising a primary jet of oxygen-rich gas and a secondary jet of oxygen-rich gas, the primary jet of oxygen-rich gas being injected close to the jet of the second fuel in order to cause a first incomplete combustion, the gases issuing from this first combustion still comprising at least part of the second fuel, while the secondary jet of oxygen-rich gas is injected at a distance I₂ from the jet of the second fuel which is greater than the distance between the jet of the second fuel and the primary jet of oxygen-rich gas closest to the jet of the second fuel, in order to enter into combustion with the part of the second fuel present in the gases issuing from the first combustion, the primary jet of oxygen-rich gas being divided into at least two primary jets: at least a first primary jet of oxygen-rich gas which is injected in contact with the jet of the second fuel, and at least a second primary jet of oxygen-rich gas injected at a distance I₁ from the jet of the second fuel, said distance I₂ being greater than the distance I₁. 15: The method of claim 14, wherein the furnace is a melting furnace in which the energy recovery means are placed on the sides and in that at least one oxy-fuel burner is placed under an air duct located in the furnace melting zone. 16: The method of claim 14, wherein the flow rate of oxygen-rich gas injected by the oxy-fuel burner is between 20 and 100% of the flow rate of oxygen-rich gas and air injected by said burner and the air duct under which it is placed. 17: The method of claim 14, wherein, for each oxy-fuel burner placed under the air duct of an operating air-fuel burner, the flow rate of oxygen-rich gas in the jet of secondary oxygen-rich gas represents 70 to 80% of the total quantity of oxygen-rich gas injected by said oxy-fuel burner. 18: The method of claim 14, wherein, for each oxy-fuel burner placed under the air duct of an operating air-fuel burner, the flow rate of oxygen-rich gas in the jet of secondary oxygen-rich gas represents 45 to 55% of the total quantity of oxygen-rich gas injected by said oxy-fuel burner. 19: The method of claim 14, wherein only the oxy-fuel burners placed opposite the energy recovery means recovering the heat from the flue gases are in operation. 20: The method of claim 14, wherein, for each oxy-fuel burner placed under the air duct of a stopped air-fuel burner, the flow rate of oxygen-rich gas in the jet of secondary oxygen-rich gas represents 70 to 80% of the total quantity of oxygen-rich gas injected by said oxy-fuel burner. 21: The method of claim 14, wherein, for each oxy-fuel burner placed under the air duct of a stopped air-fuel burner, the flow rate of oxygen-rich gas in the jet of secondary oxygen-rich gas represents 45 to 55% of the total quantity of oxygen-rich gas injected by said oxy-fuel burner. 22: The method of claim 14, wherein the combustion carried out by the air-fuel burners is substoichiometric and in that the combustion carried out by the oxy-fuel burners is superstoichiometric. 23: The method of claim 15, wherein the flow rate of oxygen-rich gas injected by the oxy-fuel burner is between 20 and 100% of the flow rate of oxygen-rich gas and air injected by said burner and the air duct under which it is placed. 24: The method of claim 15, wherein, for each oxy-fuel burner placed under the air duct of an operating air-fuel burner, the flow rate of oxygen-rich gas in the jet of secondary oxygen-rich gas represents 70 to 80% of the total quantity of oxygen-rich gas injected by said oxy-fuel burner. 25: The method of claim 15, wherein, for each oxy-fuel burner placed under the air duct of an operating air-fuel burner, the flow rate of oxygen-rich gas in the jet of secondary oxygen-rich gas represents 45 to 55% of the total quantity of oxygen-rich gas injected by said oxy-fuel burner. 26: The method of claim 25, wherein the combustion carried out by the air-fuel burners is substoichiometric and in that the combustion carried out by the oxy-fuel burners is superstoichiometric. 27: The method of claim 15, wherein the combustion carried out by the air-fuel burners is substoichiometric and in that the combustion carried out by the oxy-fuel burners is superstoichiometric. 28: The method of claim 27, wherein the flow rate of oxygen-rich gas injected by the oxy-fuel burner is between 20 and 100% of the flow rate of oxygen-rich gas and air injected by said burner and the air duct under which it is placed. 29: The method of claim 27, wherein, for each oxy-fuel burner placed under the air duct of an operating air-fuel burner, the flow rate of oxygen-rich gas in the jet of secondary oxygen-rich gas represents 70 to 80% of the total quantity of oxygen-rich gas injected by said oxy-fuel burner. 30: The method of claim 27, wherein, for each oxy-fuel burner placed under the air duct of an operating air-fuel burner, the flow rate of oxygen-rich gas in the jet of secondary oxygen-rich gas represents 45 to 55% of the total quantity of oxygen-rich gas injected by said oxy-fuel burner. 31: A system for carrying out a combustion in a furnace comprising: a) at least one air-fuel burner composed of: 1) an air duct; and 2) at least one fuel injector placed under or at the center of the air duct and perpendicular to the furnace wall; and b) at least one oxy-fuel burner placed under the air duct of the air-fuel burner and composed of at least one set of injectors comprising: 1) at least one fuel injector; 2) at least one oxygen-rich gas injector placed so as to inject said oxygen-rich gas in contact with the fuel injected by the injector of the oxy-fuel burner; 3) at least one oxygen-rich gas injector placed at a distance I₁ from the fuel injector of the oxy-fuel burner; and 4) at least one oxygen-rich gas injector at a distance I₂ from the fuel injector of the oxy-fuel burner, where I₂>I₁, said injectors of the oxy-fuel burner being perpendicular to the furnace wall. 32: The system of claim 31, wherein the injector of the first fuel of the air-fuel burner is placed under the air duct and in that said injector and the injectors of the oxy-fuel burner are placed substantially in the same horizontal plane. 33: The system of claim 31, wherein the oxy-fuel burner is composed of two sets of injectors, said sets being arranged symmetrically about the center of the air duct under which the burner is placed. 34: The system of claim 33, wherein at least one fuel injector of the air-fuel burner is placed between the two sets of injectors of the oxy-fuel burner. 